On Wed, 10 Nov 2010 17:00:27 +0800
Bob Liu <lliubbo@xxxxxxxxx> wrote:
> __set_page_dirty_no_writeback() should return true if it actually transitioned
> the page from a clean to dirty state although it seems nobody used its return
> value now.
>
> Signed-off-by: Bob Liu <lliubbo@xxxxxxxxx>
> ---
> mm/page-writeback.c | 4 +---
> 1 files changed, 1 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index bf85062..e8f5f06 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1157,9 +1157,7 @@ EXPORT_SYMBOL(write_one_page);
> */
> int __set_page_dirty_no_writeback(struct page *page)
> {
> - if (!PageDirty(page))
> - SetPageDirty(page);
> - return 0;
> + return !TestSetPageDirty(page);
> }
The idea here is to avoid modifying the cacheline which contains the
pageframe if that page was already dirty. So that a set_page_dirty()
against an already-dirty page doesn't result in the CPU having to
perform writeback of the cacheline.
The code as it stands assumes that a test_and_set_bit() will
unconditionally modify the target. This might not be true of certain
CPUs - perhaps they optimise away the write in that case, I don't know.
Yes, you're right, __set_page_dirty_no_writeback() should return the
correct value. But the way to do that while preserving this
optimisation is
if (!PageDirty(page))
return !TestSetPageDirty(page);
return 0;
This optimisation is used in quite a few places and is done in
differeing ways depending upon what is being modified. I've never
really seen any quantification of its effectiveness.
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